Non-gonococcal urethritis (NGU) was found to account for about seventy-five percent of the urethritis seen in males in a civilian venereal disease (VD) clinic population. Chlamydia trachomatis was isolated from the urethra of forty-two percent of men with NGU in this population. The prevalence of antibody to Chlamydia was found to be significantly higher in the VD clinic patients than in a control group. The prevalence of C. trachomatis and other potential pathogens also was studied in a Navy NGU population and a group of comparable controls. This study has recently been completed and the data are currently being analyzed.